What is the slot time? Why is the Github repo seemingly out of sync with the block explorer?
20 seconds. Afaik, it is specified in the genesis block, as described here: https://github.com/input-output-hk/cardano-sl/blob/master/scripts/prepare-genesis/README.md
"slotDuration" - slot duration, in microseconds
Because your link is pointing to a commit from May 29, 2017 and to a file that does not even exist anymore